Q.Which of the following would necessarily decrease the density of a population in a given habitat?

Population density in any habitat changes through four processes: natality (births), mortality (deaths), immigration (individuals moving in), and emigration (individuals moving out). Natality and immigration add individuals to the population, so they increase density. Mortality and emigration remove individuals, so they decrease density.

When natality exceeds mortality, more individuals are born than die—the population grows. Similarly, when immigration exceeds emigration, more individuals arrive than leave—again, density rises. Both scenarios in options A and B lead to an increase, not a decrease. …

Population density — the number of individuals per unit area or volume — changes constantly in response to four fundamental processes. Two add individuals to a population: natality (births) and immigration (arrival of individuals from elsewhere). Two remove individuals: mortality (deaths) and emigration (departure to other areas). The balance between these four determines whether a population grows, shrinks, or holds steady.

When natality exceeds mortality, more individuals are born than die. This produces a net gain, so density increases rather than decreases. Similarly, when immigration exceeds emigration, more individuals arrive than leave, again raising density. Both scenarios in options A and B describe population growth, not decline.

The question asks what would necessarily decrease density. Mortality removes individuals through death; emigration removes them through departure. Both are loss processes. When either or both operate, they pull density downward. Crucially, the question does not compare mortality to natality or emigration to immigration — it simply states that mortality and emigration are occurring. Even if births and immigration are also happening, the presence of mortality and emigration guarantees some reduction in density (though the net change depends on the relative magnitudes of all four processes). The phrasing "mortality and emigration" identifies the two processes that, by their very nature, subtract from population size. …

Sort all four processes into an 'adds' column (natality, immigration) and a 'removes' column (mortality, emigration) first; only an option built entirely from the 'removes' column can …